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-   -   Speed and current (http://www.wakeworld.com/forum/showthread.php?t=802267)

tn_rider 06-05-2014 8:13 AM

Speed and current
 
I got into a discussion a while back with some co-workers about speed in current. Here is my question. If you are traveling with the current in your boat doing 25 mph, and the current is moving 10 mph are you going 25mph or are you going faster than 25mph because of the current? My own opinion is that you are going faster than 25. Sorry for the stupid question but the discussion got pretty heated at the end. A few of us agreed and a few of us disagreed.

16igordon 06-05-2014 8:31 AM

Faster- 35mph. If the boat speedometer was reading 0mph, it would be moving backwards 10mph. So you would have to be moving 35mph in relation to the water. However, the boat speedometer might read 35 if its from a speedo, because of the water? However if a GPS speedometer read 25 yes you would be going 35.

racer808 06-05-2014 8:32 AM

It's all relative velocity

(4.0 m/s)2 + (3.0 m/s)2 = R2
16 m2/s2 + 9 m2/s2 = R2

25 m2/s2 = R2

SQRT (25 m2/s2) = R

5.0 m/s = R

To answer your question it would depend on wind, width of river, displacement of the water craft

racer808 06-05-2014 8:33 AM

Quote:

Originally Posted by 16igordon (Post 1879808)
Faster- 35mph. If the boat speedometer was reading 0mph, it would be moving backwards 10mph. So you would have to be moving 35mph in relation to the water. However, the boat speedometer might read 35 if its from a speedo, because of the water? However if a GPS speedometer read 25 yes you would be going 35.

Actually you would be going 30

0klahoma_Breakdown 06-05-2014 8:41 AM

You would be traveling 35 mph "ground speed". A GPS speedo would read correctly. Ground speed is a term pilots use to differentiate from airspeed. You could be traveling 150mph in the air but fighting a 50 mph head on wind and only be advancing at 100mph ground speed.

GPS speedo is a problem for a boarder on a fast flowing river though. For a river, you would want a paddle wheel speedo if you regularly rode in faster flowing waters.

WAKE_UP_TN 06-05-2014 9:04 AM

So if you were goin upstream at 35mph and the current was 35 mph you would actually be moving at a relative speed of 0? However a gps speedo should read 0? Wouldn't the GPS speedo show your speed relevant to your actual ground speed? A gps shouldn't know how much current or wind speed... It's basing it's reading off of points and time to points. A car traveling into the wind is goin 60mph... The wind is making it harder to go 60mph and the engine has to work harder but that doesn't mean it's not going 60 mph...

tn_rider 06-05-2014 9:06 AM

Quote:

Originally Posted by WAKE_UP_TN (Post 1879818)
So if you were goin upstream at 35mph and the current was 35 mph you would actually be moving at a relative speed of 0? However a gps speedo should read 0? Wouldn't the GPS speedo show your speed relevant to your actual ground speed? A gps shouldn't know how much current or wind speed... It's basing it's reading off of points and time to points. A car traveling into the wind is goin 60mph... The wind is making it harder to go 60mph and the engine has to work harder but that doesn't mean it's not going 60 mph...


But the water is moving already. If you are moving WITH the water and the prop propels the boat forward it's going to be going 25 PLUS the speed of the current added. That's where your gps will be off.

WAKE_UP_TN 06-05-2014 9:16 AM

If you pull your boat behind the truck at 35mph what will the gps say? Water movement shouldn't be a factor...

Jeff 06-05-2014 9:18 AM

Quote:

Originally Posted by WAKE_UP_TN (Post 1879818)
So if you were goin upstream at 35mph and the current was 35 mph you would actually be moving at a relative speed of 0?

Depends how you're measuring that 35 MPH. If it's a pitot or paddle wheel speedometer that says 35 MPH then yes, you'd be advancing across the earth at 0 MPH but if you jump out of the boat it will feel like you jumped at 35 MPH, just like if you had jumped into that same river from the shore. If your GPS is saying 35 MPH then you are indeed moving at 35 MPH across the earth and you're traveling at 70 MPH across the surface of the water (i.e. if you fall out of the boat it will feel like you fell out of a boat going 70 MPH)

Quote:

Originally Posted by WAKE_UP_TN (Post 1879818)
The wind is making it harder to go 60mph and the engine has to work harder but that doesn't mean it's not going 60 mph...

Correct but the road isn't moving (Relative to the earth's surface). The wind affects a boat in the same way as it would a car (for the purposes of this discussion). If the boat=car then the water=the road and the wind=wind. If you jump out of that car when you hit the road it will feel like you were going 60 MPH.

TomH 06-05-2014 9:25 AM

It just depends what you're measuring speed with, and what its relative to . If you're measuring 25 mph with GPS, that's how fast you're going from point a to b, regardless of the current. The difference is you're not doing 25 mph relative to the water, so the rider would feel like you're driving slow because your speed relative to the water is less than 25 (when traveling with the current).

Now, if you're measuring with a pitot or paddle wheel, your 25 mph is measured relative to the water, so in reality, you're going faster than 25 mph from point a to b (with the current), but from the boats/riders perspective, you're doing 25. This is why it's nice to have a paddle-wheel when dealing with fast current rivers.

WAKE_UP_TN 06-05-2014 9:26 AM

But you are missing my point... The boat behind the truck is on a trailer....the boat isn't moving itself at all... The GPS is basing it's speed off of how fast the object is moving between 2 points on a still map. Water doesn't change it... Ask a pilot. Wind behind you yes can make you travel faster... But your actual speed increases with the wind to your back... The gps reads how fast the object is actually moving...

kx250frider617 06-05-2014 9:38 AM

If you boat has a paddle wheel speedometer it will say 25mph, if it has a gps speedometer it will say 35mph. I think everyone is over thinking it. If your towing a rider, I hope your adjusting your speed according to the river speed.

However, if you are ever wanting to learn new tricks and are riding on a river, drive the boat up river. It feels like you going so much slower. It's a little mind game.

Jeff 06-05-2014 10:08 AM

Quote:

Originally Posted by WAKE_UP_TN (Post 1879826)
But you are missing my point....

I know I am.

Jeff 06-05-2014 10:29 AM

Forget about wind for a minute. Think of running on a treadmill going at 10 MPH. If your're running on it you'll be doing just about every bit of work that it would take you to run 10 MPH on the surface of the earth. You'll be burning calories at the same rate and your muscles will feel like they're propelling your body at 10 MPH.

How fast are you going? The answer depends on your perspective. A GPS would say you're not moving. Your quads, calves, lungs, heart, etc would beg to differ and say 10 MPH. Both answers are correct given the lack of specificity in the original question of "How fast are you going?". Relative to the belt on the treadmill you're going 10 MPH. Relative to the surface of the earth you're going 0 MPH.

Now move back to the boat (Still forgetting about wind). The river current (The treadmill) is 10 MPH headed south. Your boat is going north and the paddle wheel based speedometer indicates 10 MPH. The engine is running and consuming fuel and working as if you were moving at 10 MPH.

How fast is the boat going? Just as before, the answer depends on the perspective. The GPS would once again say you're not moving. The engine, transmission, prop and your fuel gauge movement would beg to differ and say 10 MPH.

Now, if you add in the variable of wind resistance it becomes much more complicated relative to the "feelings" of your quads, calves, lungs, heart, etc in the treadmill example and the "feelings" of the engine, transmission, prop and fuel gauge movement in the boat example. It does not have an impact on measured speed relative to the earth or the treadmill belt/water in either case but rather just the amount of energy required to maintain a given speed.

baitkiller 06-05-2014 12:53 PM

SOG /
StW /

Speed over ground
Speed through water

Remember, everything is relative, especially your relatives.

ottog1979 06-05-2014 1:22 PM

It blows my mind why this subject is such a mystery or perceived as complicated. Jeff D's treadmill example is a perfect analysis/explanation. GPS will measure speed over the surface of the earth (land or water). On a river, you will have to add or subtract (depending on up or down current) the current speed from the GPS reading to get your accurate over-the-surface of the water speed. Paddlewheel doesn't have to do this because it is already measuring your speed relative to the water (regardless if the water is moving or not).

If you're sitting in the river water waiting to be picked up and watching the shore go by at the speed of the current are you moving or not? Relative to the water - NO. Relative to the land (surface of the earth) - YES, at the speed of the current. Simple.

zimme 06-05-2014 2:58 PM

But... But... But... could a plane take off if the runway was a giant treadmill???????????

Fiveflat 06-05-2014 3:20 PM

In the river I put my boat in neutral last night. My GPS read 3mph. When I dropped an anchor and stopped moving with the water flow my gps read 0MPH

0klahoma_Breakdown 06-05-2014 3:55 PM

Quote:

Originally Posted by zimme (Post 1879873)
But... But... But... could a plane take off if the runway was a giant treadmill???????????

If the treadmill was the air, and if it was blowing faster than the stall speed of the aircraft...yes :D

FastR3DN3K 06-05-2014 4:45 PM

http://i1.kym-cdn.com/photos/images/...kie-chan-l.png

spencercoon 06-05-2014 5:38 PM

I couldn't bear to read all of the mumbo jumbo above so if this has already been stated I apologize. To answer the original post, you would be traveling at 25mph in relation to the water that the boat is in. If you were using the land as your point of reference, your speed would be 35mph in relation to it.


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